Recent sedimentation rates across the Norwegian Trough
HELMARKUNZENDORF,ODDVAR LONGVA&MATIHIA5PAETZEL
Kunzendo rf, H.,Longva,O.& Paetzel, M.1996:Recentsedi me ntat ionrates acrossthe NorwegianTrough .Nor.geol.
unders.Bull.430, 67-74.
Fromthe systematicsamplingof 20statio nsofthe Norwegi anpart of the 5kagerrakin 1993, sixcores werechosen fordeterminati on ofrecentsedim ent atio nrates bythelead-2 10met ho d and verificatio ns byCs-137 depth distribu - tion s.Byusingthegamma- spectr ometricvariantof the method,bothnatural andant hropog enic radioi soto pesare sim ulta neously measuredinthesam ples.
Thehighest average linear sedi me ntati on rate (4.7 mm/year) wasfoundfor a core(NC-84)fromst at ion67,on the southe rn lowerm ost slope oftheNorwegianTrough ; arelativelyhigh averagesed iment atio nrat e(2.3mm/year) was also deduced fromthe measured dataforthecore(NC-13) from stat ion 56,taken initscent ral(deepest) part.
Thecores from the nort hweste rn partof thetroug h, from statio ns 69 (NC-97)and74(NC-134),showed ratesthat were closeto or somew hathigherthan 1 mm/year, whilecores from stat ions65B(NC-71)and71(NC l 13)showed significant lydisturb ed dept hprofi les.
Using unsuppo rte dlead-210 inte nsities and the physical dataonthecores (waterconten t,dry weig htand bulk den- sity of the coreslices)itwas possible torecon structthesed imentat ionhistoryof the cores.Whenthemod ell ing (const antrateofsup ply)is appl ied tothe Cs data,itis obv ious that Cs-137 distribu ti ons wit hdepthcanmost likely be explained bythree superimposed ant hro poge nicevents:Cherno by laccident,5ellafie ld release and nuclear bomb testing,withtheSellafieldreleasebeing themostdomina t ing feat ure observedin theinvest igatedcores.
Helmar Kunzendorf,Rise NationalLaboratory,Envir onmental ScienceandTechnologyDepartment,P.G.Box49,DK-4000 Roskilde,Denmark.
Oddvar Longva,TheGeologicalSurveyofNorway,P.G.Box3006,N-7002Tron dh eim,Norway.
MatthiasPaetzel,Universityof Bergen,Geologica lInsti tu te,P.G.Box AII/?gaten41,N-s007Bergen,Norway.
Fig.1.Investiga tedareain theNor wegian partoftheSkagerrak showing positionsofsedime ntcoresusedfor lead-270 dating.
initiatives, as exemplified by the chemical data.In other words,the chemical studies of sediments strongly need some sort of dating tool.
Many of the pre-1990sediment core geochemical data were published without application of any dating tool, largely because of the timely and expensivedating met- hods available in the past. Most of them were checked for disturbances by experienced sedimentologistsbutsince many disturbed sediment cores show no macro- or microscopicfeatures which provideinfor mat io n on these disturbances, a convincing depositional check of the sediment cores is lacking.
Introduction
The systematic investigations in the Norwegian part of the Skagerrak and the transitional area to the North Sea by the Geological Survey of Norway and the University of Bergen alsoaim at the evaluation of sedimentgeochemi- cal features which hitherto have not been published in the form of geochemical maps.Relying on the distributi- on of chemical elements along a sedimentcore requires, however, that the core has been checked for discontinui- ties in sediment deposition,i.e.for bioturbationor physi- cal mixingby currents, slides or human activity.If this had not been done,most of the geochemicaldata would be more or lessworthless for the following reasons.
Supposing that the along-corechemicaldatasuggest an accumulation or/and depletion of elements in the upper few cm ofthecore,the questionthen arises whet- her this isby normal diagenetic changes,caused by pollu- tionor by redoxmobilisation, respectively .The observed chemical pattern,however,could also be caused by a dis- turbance insedi ment deposition at some point in time. These processeshave been studied in great detail over the past years and the resultsof such studies have been discussed in numerouspublications.In any case,a proce- dure to determine sudden or long-lasting changes in the sediment deposition pattern is needed and, furthermore, chemical depth profiles in recent sediments need to have a time scale to evalute whether anthropogenic input is increasing from a certain period of time or whether there is a significant pollution decreasecaused by public law
9'E 11'E
59'N
57'N
68 He/mar Kunzendorf, OddvatLongvo & Morrhio5 Poerze/
The only availab le radiometric dating method for recent sediments is based on lead-210,which has ahalf- lifeof 22.3 years.Thisradioisotope has been appliedindi- rectly for dating purposes over the past few decades through its daughter isotope Po-21O.Because the physi- cal method forthe determ inat io n of this radi oisotopeis by alpha-spectrometry,tota l dissolut io nof sam pleshas to be carried out (e.g., Robbins & Edgingto n 1975, Robbins 1978, Madsen & Seren sen 1979, Christensen 1982).Sincethemid-eightiesamore directtechniq uefor lead-210dating hasbeen developed,where the leadradi- oisotopeis determined directlyin anuntreated sediment sam ple by gamma-ray spectrometry (e.g., Joshi et al.
1988),and appli cationof thetechniqueto Katt egatsedi- ment s hasalsobeenreport ed(Christ iansen etal. 1993).
In the present paper we repo rt on the dating of six Skagerrak cores bythenew technique.We concentrate on the datainterpretati on ofthesam ples and use sim ult- aneously generated Cs-137 data to verify the dating.
Averagelinear sedimentationrates and massaccumulati- on rates have been estimated.For the coresthat were dated,the con tents and changes in some chem icalele- mentswit h dept harealsobriefly discussed.
Geolog ical setting
The Skagerra karea is at present one of the focal areas whenst udying anthropogenicchemicalfluxeson the sea floor. The hydrograph ical settin g ofthe area hasbeen dis- cussed by, e.g., Svansson (1975), Rohde (1987) and Hognestad(1987).
Thesediment cores were taken duringa cruisein1993 by RN 'Hakon Mosby'as part of asystemat ic investigati- on of marine sediments in theNorw egian Skagerrakarea.
This proj ect includes both systematic geophysical and geological work. Starting at the easternmost part ofthe Norweg ianSkagerrak,the projectispresently conti nuing alon gthe south western coast al area of Norway.The posi- tionsof thedated coresaregiven inTable 1 andplotted in Fig.1.
Allofthecoreswere taken by Niemistb coring equip- ment,recovering up to 0.6m long cores of diameter 56
Table 1. Coring stations and sediment core characteristics.
Core No Station Position Water depth(m)
NC-13 56 58.15480N 252
9.28960E
NC-71 658 58.05560N 340
9.66730E
NC-84 67 57.96950N 483
9.18880E
NC-97 69 58.08610N 640
8.94980E
NC-113 71 58.20 160N 307
8.70630E
NC-134 74 58.0230N 594
8.833S-E
NGU-BULL430,1996
mmdirectly in an PVCcylinder.Parallelcor eswere taken at all stations except for station 65B.From each stati on, one core was usually X-rayedand a sedimentological des- cription of the core was carriedout.
In thedeeper parts oftheSkagerrak,thesediment sare generally very fine-grained with aclay cont ent excee- ding 50% (B0e et al.this volume).Above 300 m water depth onthe Danishslope,sandy sedimentsprevailand along the Norwegiancoast they vary from claysto tills.All the coresshow significant,but generallyverymoderate bioturba t ion.
Core NC-113(stat ion 71),closeto theNorwe gian coast, had a clay contentof 50-60%throughout the core.The upper 15 cm seemed to be severelydisturbed(B0e 1994).
Cores NC-13,NC-97and NC-134(stations,56,69 and 74, respectively) are from the more central parts of the SkagerrakDeep(Fig. 1),wherethe thickness of Holocene sedi ments varies betwe en 10 and 20 m (Riseet al. this volume).The upper 5 to 10 cm(2.5Y3/2)are usually very soft,and the cores are moderate ly bioturbated.Core NC- 97is morefine-grained than the othertwo coresfrom the deep part sand is characterisedbyclaycontentsof more than70%throughoutthe core. Core NC-84(station 67is from the low er part of the Danish slope, where the Holocene sedi ments are betw een 40 and 50 mthick(Rise et al. thisvolum e).Thesand content is nearly zero and the silt content increasesto 60%at adepth of 30 cm, which indicates a change in the sedi ment ary environ - ment.
Methods
The sediment cores were cut intoslices of 5 to 10 mm thickness(t hethi nner slices were used for near-surface sedi ments).Wetsedimentswere weighed and dried,and the dry weigh twasthen determ ined. Through thisproce- dure,boththewater content ofthesedimentdiskandits bulkdensit y(Rhobulk=dryweight/wetvol ume)couldbe calculated.
Dried sediment material (10 to 30 g) wasthen filled into aluminium containers with thin plast ic foil bott oms and tops.The containerswere sealed by tapeand then storedfor at least 14 days to restore secularequilibrium which maybedisturbed due to radon escape. However, this routineoperationis regarded as of minor importance becausethe marine sedimen ts in generalalready suffer from chemical separations in sea water which usually leads tosig nifica ntradioactive disequilibrium.
Thesam ple container areaof 72 mm diameterisrelati- vely large,4072 mm', which reduces physical radiati on interferencescaused by varying particlegrain sizes.As a rule,more or lessthe same amount of samplematerialof slicesfrom a core were used for count ing.In thisway, possible self-absorptioneff ect sin the sampleare reduced con siderably. This is not always necessary, however, because expe rience has shown that there is a limited density variation of sedi mented material over the relati- vely short time spans involved (several hundreds of
NGU-BULL430,1996 He/mar Kunzendorf, Oddvat Longva& Matthias Paetze! 69
Cs-137 ( cps/kg) L ead-210
uns 0
NC-71 100
200
300
400
500
600
0.1 0.5 1 10
0
NC-11 3 80
...
E
160
-
E s:Q.ID
240
0
320
400
0.2 0.4 0.1 1
Fig.2.DepthprofilesofCs-137and unsupportedlead-210of coresNC-71and NC-/13.Thestipp ledlinein thefigureindicatesthe level for an inferredback- ground.Both coreswereexcludedfrom the dating becauseofpossiblesedime nt depositiondisturbances.
years).
For gamma-spectrometric purposes, the container s were then measured in a low -level radiat ion counting facility.A reverse-elect rode coaxialGe detector(10% reI.
efficiency)wit h energy resoluti on valuesof640 eV and 1.7 KeV at 5.9 KeV and 1332 KeV,respectively, was used.
The gamma-rayspectra from the samples were collected by means of a normalnuclear counting apparatus(stan- dard counting electronicsand multichannelanalyser).
To reduce the backgroundradiation ,the counting sys- tem was shielded by at least 100 mm Pb around the
detector head. Because the present R is0 counting facility
is positioned in a normallaboratory building ,background
counts could not be reduced to values lower than about 5.8counts per second (for the range 0 to 680 keV). In general,background variationswith time,obtainedfrom repeated measurements were usually below the 10 per cent level. However, it should be mentioned that the actual background level for a sample consisting of 0-20g sample material of varying physical and chemical compo- sition is difficult to determine. This is because back- ground depends on the composition of the sample matrixmaterial; hence,the background obtainedfrom an empty sample container is usually too high because of the lack of an absorbing samplelayer of the same height as the actual sample.
70 He/mar Kunzendorf, OddvarLongva& March/osPaetzel
Table 2.Estimatedsedimentation(R)and sediment accumulation(w)rates for the Skagerrakcores.
CoreNo Depthrange R Massdepthrange W (mm) (mmyear- t} (g cm·2) (g m-2year-l)
NC-13 15-155 2.3±0.4 0.5·3 449181
NC-71
NC-84 25-95 4.7±1.8 1-5 843±198
NC·97 5-95 1.1±0.1 0.2·2.5 278±34
NC-113
NC-l34 5-95 1.5±0.1 0.2-2.5 365±37
All the lead-210 intensit ies(cps/kg dry weight) were calculated directlyfrom the counting rateof the 46.5 keV peak.Cs-137was determined from its 661.6 keV peak.A numberofot her nat uraldecay seriesandant hropogenic radioi sotope intensiti es were also determined,partly to checkthe equili brium statusof the sedi ment disksor to discriminateagainstpollution ofthe sediments.Counting time s wereusuallyinexcessof 80000s.Atsuch count ing intervals,counting errors for lead-210 wereusually below 10%butweregreater(10 to 20%)for sedimentstakenat depth.
Theintensity ofunsupported lead-21O was calculated by first subtractingaconstant backgroun d(data from an empty sample container used) and then the equivalent supportedPb-210intensity.Using anempty samplecon- taine r forbackgroun d subtracti on is not the correct way to obtain netintensit ies becauseitis the non-radioactive matrix thatdetermines the amoun tof background radia- tion reachingthe dete ctor.However, as afirst approxima- tion,this calculatio n strategy issufficient.The supported lead-21Ointensity is generally assumed to be proportio- nal to that of Ra-226orBi-214 or anyother radioisoto pe observed in the decayseries.The gamma-spectr omet ric method allo wsthe direct deter minationof Ra-226 by its 185 keV, although some interference from U-235 may occur. An experimentally determ ined fracti on of these radioisotopesfrom a rock samplein certificatedradi oacti- veequilibriu m wassubt racted from the total lead-21O activity,andthe remaining activity is namedthe unsup- ported lead-21O.Bydoingthis,thegamma-spectrometric method must beregard ed superior to theconventional alpha-spectrometrictechniquebecause it alsogivesvalu- es for the supportedlead-21O direct lyfrom the measure- ment ofsome oth erradioisotopes,e.g,Ra-226or Bi-214.
A possible problem with self-absorption, due to the low-energy radiation of lead-210, can be handl ed by using similar sam ple size and filling grade for all of the sample containers, Le. keeping these parameters con- stant within±5%.An internal density-check procedure, however, is present ly being describ ed (Kunzendorf, 1996)whichinvolves theuse,fo rcorrect ionpurposes,of other , simultaneously determined gamma-rad iat ion also present inthe sediments.
NGU-BULL430.1996
Results and discussion
Lead-210dating
Unsupported lead-21O and Cs-137 data are plotted vs.
depthinFigures 2 and 3.Thedatingresults aregivenin Table2. From Fig.2it canbe seenthatcoresNC-71(stati- on 65B)andNC-113(station 71)are difficultto use for age calculations. While the latter core is clearly disturbed dow n to200mm depth,there is some trendin the Cs-137 data of coreNC-71, whichboth show a peak at dept h(ca.
250mm)and generally high intensiti es.Ifoneargu esthat theelevated Cs-137 is due to Sellafield releasespeaking in the mid-sevent ies,arelatively highsediment ation rate, close to 10 mm/year,would apply. If thiswere the case, sedimentdatain excess of 600 mm depth are needed to det ermine the rate by lead-21O data.The core history evaluations were therefore not carried out for these cores.
All the other cores show very clear depthprofilesfor both radioisotopes (Fig. 3).The curves forunsupported lead-21O(Fig. 3) suggest that there is some variati onin sedimentation rate in the Norwegian Trough. When usingthe constantinitialconcentration model(CIC)(e.g., Robbins19 78),averagesedimenta tionra t esforcore sNC- 13,NC-84,NC-97and NC-134(stations 56,67,69 and74, respectively)are 2.3,4.7,1.1 and1.5mm/year,respective- ly(Table 2).Avariati on ofsediment ationratesisexpected because the thickness of the Holocene depositsis quite variableinthis area(Riseet al. thisvolume).Some of the above rates,especially thosefrom the centralparts of the trough,e.g.cores NC-13and NC- 134,areingood agree- mentwith thefindings of van Weering et al.(1987, core 11and 24,respect ively).Theseauthors also reported a number of varying rates in the Skagerrak area (van Weering et al. 1993).A major feature of their lead-21O depth profil eswas that sedimentmixing in the upper few tensof cm wasdedu ced,becauseconstant unsupported lead values were observed in the uppermost sediment layers. Anim portant finding was thattheirdata alsosug- gested that sedimentation ratesin theSkagerra kincrease eastwards.However,to depictthe sedi mentation pattern in the Skagerrakareait isclearthat much moreclosely spacedundi sturbedsedimentdata are neededtoevalua- te the pertinent hydrog raphicregimeinspaceandtime.
Incorporating the bulk sediment density data along the cores(rbulk
=
dry weight/wet vol ume),averagesedi-ment accumulation ratescan alsobecalculatedfrom the constantinitialconcent ration model.Thesedatafor cores NC-13,NC-84, NC-97and NC-134 were estimated to be 449,843,278 and 365 g rn' year', respectiv ely. Again, these valuesare varyin g conside rablyandthey require an explan ation ofthe basic sedimenttransportsystemin the Skagerrak and the adjacent water masses (Nort h Sea, Kattegat).However,the accumu latio n rateof especially core NC-13agrees wit h that presented by Paetzelet al.
(1994,coreA).
By using the constant rate of supply model (CRS) for
NGU-BULL 430.1996 HelmarKunzendorf,OddvarLongva& MatthiasPaetzela 71
NC-13
0
100
E 200
.s
s:Q.Cl> 300
Cl 400
500
0.2 Cs-13?
0.4
(cpslkg)
0.1
Lead-210 uns 0
100
E
NC-84
.s
200.5C-
CD 300
Cl
400
500
0.2 0.4 0.1 1
Cs-13? (cps/kg) Lead-210
uns 0
100
E 200
.s
s:Q.
Cl> 300
Cl
400
·500
0.2 0.4 0.1 1
Cs-13? (cpsl kg) Lead-210uns
0
100
NC-134
E 200
.s
s:Q.
Cl> 300
Cl
400
500
0.2 0.4 0.1
Cs-13? (cps/kg) Lead-210
uns
Fig.3.Depth profiles ofCs-137and unsupport ed lead-2 10 of coresNC-13,NC-84,NC-97and NC-134showingclearactivitydecreaseswithdepth forlead-2/O.
72 He/marsuozendort. OddvatLongva&Ma thiosPaetzei
0-.--- - - ,
GU-BULL 430.1996
0 -.-- - - -- - - -- - - ---,
NC-13 NC-84
100
100
200
200
1980 1940 1900 1990 1970 1950
0 0
NC-97
~ NC-134
50 50
100
1990 1950 1900
100
1990 1950 1900
Fig.4.Resultsofage estimationusingthe CRS modelfor coresNC-13,NC-84,NC-97 andNC-134.
Sellafield release
NC-1 3 N C-84 NC-97 NC-134
1990 1980
L..
~I1l
1970
1960
1950
2 4 0.2 0.4 0.2 0.4 0.2 0.4 0.4 0.6
(P Bq) (cps/kg)
Fig.5.Dateddepth profilesofCs-137ofcoresNC- 13,NC-84,NC97and NC-134.
thePb -2 10 dating(e.g.,Ro bbi n s1978),th e sedimenthis- toriesof thefour corescan be estim ated (Fig.4). These data weregat hered by applyin gthemeasured unsuppor- ted Pb-210int ensit ies to acore depth where backgroun d levels were reached. At depth,lead-210 data from the (I(
fit wereused. The ageofa sedi ment at any depth can
then be calculated from the equation t(x;l = 1-' In (AtotaI/A(xj))where Iis the Pb-210decayconstant (year'), AtotalthetotalPb-210 activity along the core (in 10-3cps crrr-)and A(xj)is the activitybelow sediment depth
x,
A problem that often arises when constructing the sedi- mentation history is that of the lack of bulk density dataNGU-BULL430,1996 He/marKunzendorf,Oddvar Longva &Marrh/asPaetze/ 73
Fe
4 I;
(pp m)
4 500 900
4
6000 4 5
Fe
2000
2000 6000
(ppm) Mn 5 20 00 8000
(%) 3
Sr
'00 200 (p pm)
~
, l ~~ 5-
«: ).
-;?$
Fe
~
Sr
"f
, ?Mnj $ K
, ,. ~
200' 300
OTN:.:.C...:.-'.:::34=.y.--r_=_-,-::::::==~--r--"I'l NC -'3
o
200
200 300 " 4'i; ' 30 0 500' :2 '
NC-97
,, :, 1 , )
400 [ a
~
E E
400
200 300 3
oNC -84
200 400 s:C. 200
Q C
Fig.6.Profil esoiSr, Ca,Mn,andFewith depthfromcoresNC- 13, NC-84,NC-97and NC-134.For core NC-84,theprofileofBa with depth is also shown.
the lit eratu re leads to the following exp lanat io nof the observed Mn and Fe profilesofthe cores.It is assumed that,below the redox boundary of about 50 mm (cores NC-97 and NC- 134), manganese and iron phases in the sediment s are disso lved and the solut io ns migrate upw ard s.When reaching the oxic sediment zone « 50 mm),Mn is precipitatedpreferentiallyfrom thesesoluti- onsin the form offineoxyhyd roxideparticlesleadi ng to a sig nifi cant enrichment of this metal towards the bot- tom/seawater transit io nzone. Ironisalsoenriched in the oxic zone, but iro n diagenesis is less pronounced. Interest ingly,the hig hest concentrat ion sof Mn, both at depth (2000 compared to 1000 ppm)and inthe surface layers(up to1.4%compared to 6000),are found in core NC-13from thecentralpart of thetrough. These mecha- nisms are here displayed onwell-dat ed sediments and therefore increasethe signifi canceofint erpret ation.
While therearegenerallyrelatively low Mnconcentra- tions in thesediment s of core NC-84, the main characte- rist icis a sig nifi cant enr ichment of Ba in the upper 100 mm wit h the hig hest values at thesedim ent/seawater inte rface. Enrichment of Bastart s at some timeafter1970 and one couldarguethat core NC-84iswithi n asediment materialtransport channel transporting North Sea mate- rials to the Norw eg ian Trough. Barium could then be ascribed to hydr ocarbon explor at io n and exp loitation act ivitie s which have increased signifi cant ly since the
Core No Chernobyl Sellafieldreleases Bom btestin g
1986 max.1975 max.1 963
NC-13 + + (+)
NC-71 disturbed
NC-84 (+) + (+)
NC-97 (+) (+) (+)
NC-113 disturbed
NC·134 t» + +
Afterthe reconstr ucti on of the recent sed iment ati onhis- tory by the CRSmodel, adate can be ascrib ed to each mid-point of asedi ment coresection. Replottingthe Cs- 137data (Fig. 5)perm itsthe assignmentof Cs-137to one of the three known event s, Le. Chernobyl,Sellafield or nuclear weapons testing.Dataforthese events based on monitoringarealsoshow n in the Figure.
From Fig.5 it appears that there is a clear Sellafield com ponent in the Cs-1 37 curve sobserved for cores NC- 13, NC-84and NC-134.ltcanalso be argued that there is a contrib ut io nofChernobyl derivedCs in thesecores.Due to the relat ively low sedi mentati on rate, the dept h distr i- bution of Cs-137for core NC-134 is somew hat broader than for the othercores.The contribution from Sellafield releases, however, seems to be highe r becauseof the hig h Cs-137 values. In this respect, the sequence of Sellafield release infl uenceis NC-134, NC-84 andNC-13, which may be related to the distance to the North Sea pathways. It is alsoworthwhile to note that,in general, the cores also suggest the accumulati on of Cs-137from nuclearbomb testing,peaking in 1963.
Dated chemica l profi les
at depth.This is because core length, in many cases,is about 300 mm which is too short for dating purposesfor a sediment deposition with rates exceeding about 4 mm/year. Thisproblem did not,however, play a rolein the evaluationof the present cores which all exeeded 450 mm.
Table 3.ObservedCs-137intensitiesin theSkagerrak coresascribedtothe influenceof different events.
Cs-137 data
Selected chemicalprofile salongthe dated cores(NC-13, NC-84, NC-97 and NC-134)are given in Fig.6.These data weregenerated by a semiquantitativ e,automat ic,ener- gy-dispersive, X-ray fluorescence spect rom eter (e.g., Kunzendorf1979).
Alt houghitisnotwithinthescopeof thispaperto dis- cussthegeochemistry in detail,themost st rikingobser- vat ionis that onlythree of thefour datedsediment cores showaveryclear diageneticMn enrichment in the upper sedi ments.Themanganese profi leof core NC-84from the SE flankof the NorwegianTrough displaysa different dia- genetic imprint. Thewell-know n chemi cal behavi our of Mn andFein marine sediments described elsew herein
74 He/marKunzendorf,Oddvar Longva& MarrhiasPaetzel NGU-BULL430,1996
mid-seventies.This is also verified by the observed hig h sedimentation rate(4.7 mm/year )and a strong Sellafi eld release imprintin the sediments.However, otherproces- ses of Ba enrichment in the surfacesediments may have been operating.
Con clus ions
The dating of six sediment cores from the Norwegian Trough by the Pb-210 method suggeststhe following:
1)Average linear sedimentation ratesvary between 1.1 and 4.7 mm/year ,with the highestsedimentationfound in core NC-84(st at io n 67).Average massaccumulation rates were found to vary between 278 and 843g cm-2 year'.A sedimentationhistorywasconstructed for these coresby applying the constant rate of supply model.
2)Forcores NC-71 (st at ion65B) and NC-l13(stat ion 71), the Pb-210 and the Cs-137 activity depth profile sshowed no signifi cant variations, suggest ing that these core are greatly disturbed.
3)By assigni ngactual datesto the cores with significant Cs-137profiles it appearsthat the profiles can be explai- ned by three superimposed anthropogenic events, namely the Chernobyl accident, the Sellafield releases and the nuclear bomb testing s. It appears that the Sellafield releasesinthe lateseventi es had a major effect on Skagerra ksediment deposition.
In general, our investigations also suggest that the lead-21O dating technique based on recording gamma radiation activity with depth of several radioisotopes instead of the conventional alpha-spectrometryisa pro- mising technique in high- resolutionmarine geochemical studiesof recent sediments.Because lead-21O data may giveerroneou s result s,the Cs-137recor ded at the same time usually confi rms and strengthensthe data obtained from lead-21O. Althoug hcount ingtimes arecomparable to conventionaltechnique s(alphaspect rometry),ease of operatio n andthepossib ilit yof repeatednon-destruct ive measureme nts are the main advantages of thegamm a- spectr om etric techniqu e.Most of the physicaleffe ct s(e.g.
self-absorption)canalsobe controlled.
The increasing demand for elucidating the pollution hist o ry and the rece n tenvironm enta lch ang esindep osi- tional marinebasinsfavoursthe use of rad iometri c profi- lingas abasictool inmarine environmenta l stud ies dea- ling withchemica lelement fluxes.
Acknowledgements
Weacknowledge thehelp of thecaptai n and crewof RN'Hakon Mosby' during the samplingcampaigns.Theskillful workof P.Serensen,Riso,in carryingoutthe analysesis greatlyapp reciat ed.
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ManuscriptreceivedMarch 7995;revisedversionacceptedJuly 7995
